Soft start a c motor control

ABSTRACT

A soft start control for a delta-connected motor or an ungrounded Y-connected (star-connected) motor or the like polyphase system. To reduce the accelerating torque of a polyphase motor and thereby obtain a soft start characteristic that reduces stress and strain on the driven parts until normal running speed is reached, a gated A.C. switching device such as a triac is used in the power lines of the polyphase A.C. source. A simple time delay firing circuit connected across each triac controls the latter so that current flow is interrupted in each of the phases for a predetermined period of time during each half-cycle in repetitive sequence, the net effect being to reduce the effective voltage applied to the motor. After a predetermined time of reduced voltage acceleration sufficient to bring the motor up to or near running speed, a timer controls the firing circuit to apply full line voltage to the motor.

United States Patent Hansen June 26, 1973 SOFT START A.C. MOTOR CONTROLPrimary Examiner-Gene Z. Rubinson [75] Inventor: James E. Hansen,Milwaukee, Wis. Aflomey-nugh Rather [73] Assignee: Cutler-Hammer, Inc.,Milwaukee, [57] ABSTRACT Wts. A soft start control for a delta-connectedmotor or an 22 Filed; 20 7 ungrounded Y-connected (star-connected) motoror the like polyphase system. To reduce the accelerating [21 1 Appl'210'023 torque of a polyphase motor and thereby obtain a soft startcharacteristic that reduces stress and strain on the 521 (1.8. CI318/227, 318/230, 3l8/4l6 driven Parts until normal running P isreached, a [51] Int. Cl. H02 5/40 gated Switching device Such as a'lriacis used in [581 'Field of Search 318/227, 230, 231, the Power lines ofthe polyphase A-csource- A Simple 31 41 time delay firing circuitconnected across each triac controls the latter so that current flow isinterrupted in 5 References Cited each of the phases for a predeterminedperiod of time UNITED STATES PATENTS M during each half-cycle inrepetitive sequence, the net 3 189 810 6/1965 M G effect being to reducethe effective voltage applied to ac regor 3,652,924 3 1972 Dieterich etal. 318/227 x the After a preietermmefi reduced 3 573 580 4/1971 ageacceleratlon sufficlent to bring the motor up to or Shinozaki 3 l8/227near running speed, a timer controls the firing circuit to apply fullline voltage to the motor 12 Claims, 3 Drawing Figures M wtc SOFT STARTA.C. MOTOR CONTROL BACKGROUND OF THE INVENTION Soft start motor controlshave been known heretofore. Techniques such as resistors or reactors inseries with the motor have been known. It has also been known to useSCRs in reverse-parallel connection in one phase of a polyphase system;however, this has the disadvantage that the motor would be veryunbalanced. Furthermore, SCRs have been used in reverse-parallel in eachphase of a three-phase system, but these systems have required complexfiring circuits for control of the six SCRs.

This invention relates to improvements thereover.

SUMMARY OF THE INVENTION This invention relates to a soft startpolyphase motor control.

An object of the invention is to provide an improved soft start controlfor a polyphase A.C. load.

A more specific object of the invention is to provide a simplified softstart control for a delta-connected motor, ungrounded Y-connected motor,or the like.

Another specific object of the invention is to provide simplified firingcircuits for bidirectional thyristor means in a plurality of phases of apolyphase system.

Another specific object of the invention is to provide improved andsimplified firing control circuits for the bidirectional thyristor meansin the respective phases of a polyphase A.C. powered circuit.

Another specific object of the invention is to provide improved andsimplified firing control means for the bidirectional thyristor means inthe respective phases of a three-phase system supplying adelta-connected or ungrounded Y-connected motor whereby to providetimed, reduced-torque acceleration.

Another specific object of the invention is to provide a soft startcontrol for a polyphase load that has a small size, very low power orheat dissipation, adjustability, and low cost.

Another specific object of the invention is to provide a soft startcontrol of the aforementioned type that may also be used as an on-offcontrol for the load, allowing the load to be turned on or off from alow power signal.

Other objects and advantages of the invention will hereinafter appear.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a softstart A.C. motor control system constructed in accordance with theinvention;

FIG. 2 is a circuit diagram ofa timer usable in the system of FIG. 1;and

FIG. 3 is a graph showing operating characteristics of the system ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there isshown a soft start A.C. motor control system according to the invention.This system is powered from a three-phase A.C. source through powersupply lines L1, L2 and L3.

This system comprises three like motor power control circuits 2, 4 and6, one for each phase of the threephase source. As shown in FIG. 1,power control circuit 2 is connected between line L1 and conductor A.Power control circuit 4 is connected between line L2 and conductor B.And power control circuit 6 is connected between line L3 and conductorC. Since power control circuits 4 and 6 are similar to power controlcircuit 2, they have been shown schematically as rectangles to avoidcomplicating the drawing.

In this system, conductors A, B and C are connected to the threeterminals of the three-phase load which in this case is adelta-connected AC. motor M. at an ungrounded Y-connected motor could becontrolled is this system, or other like three-phase load. Conductors A,B and Ccorrespond to waves A, B and C, respectively, in FIG. 3 ashereinafter more fully described.

As shown in FIG. 1, power control circuit 2 comprises switching meansfor interrupting current flow from line L1 to the motor for anadjustable period of time during each A.C. half-cycle in repetitivesequence. This means comprises a bidirectional thyristor device such asa triac 8 connected at its main conduction terminals A2 and A1 betweenline L1 and conductor A.

A dv/dt control circuit comprising a resistor 10 and a capacitor 12 inseries connection is connected across anodes Al and A2 of triac 8.

A noise suppression network comprising a resistor 14 and a capacitor 16in parallel connection is connected between gate G and anode A1 of thetriac to suppress noise voltages that might be picked especially as aresult of connecting the firing control circuit from a remote locationthrough long conductors to the triac at terminals 18, 20 and 22.

The firing control circuit for the triac comprises means for applying atrigger pulse to the gate of the triac a predetermined, timed intervalafter the triac stops conducting. This means comprises an RC circuitcomprising adjustable resistor 24, fixed resistor 26 and capacitor 28,and a symmetrical trigger diode comprising a diac 30. As shown in FIG.1, the aforementioned terminals 18, 20 and'22 are connected to anode A2,gate G and anode A1 of the triac, respectively. Resistors 24 and 26 andcapacitor 28 are connected in series in that order from terminal 18 toterminal 22, there being a normally open contact 1R1 of start relay 1Rbetween terminal 18 and resistor 24 for closing this RC circuit. Thejunction between resistor 26 and capacitor 28 is connected through diac30 to terminal 20 and, thus, to the gate of the triac.

The firing control circuit also comprises means for shunting the same toallow full conduction of the triac throughout each half-cycle. Thismeans comprises a resistor 32 connected in series with a normally opencontact 2R1 of full voltage applying relay 2R between terminal 18 andterminal 20. As will be apparent, closure of contact 2R1 completes afiring signal path from line Ll through the gate-anode A1 junction ofthe triac to fire the triac at the beginning of each current halfcycle.I

The firing control circuit also comprises means that helps to eliminatefalse commutation due to the rapid voltage rise characteristic, thelatter being present because zero current crossover does not correspondto zero voltage due to the inductive nature of the load. This meanscomprises a resistor 34 connected in series with normally open contact1R1 across the triac anodes Al and A2.

Power control circuits 4 and 6 are similar to power control circuit 2hereinbefore described. Relay IR is provided with two additional contactsimilar to contact 1R1, these two additional contacts performing similarfunctions in circuits 4 and 6, respectively. Relay 2R is provided withsecond and third contacts for circuits 4 and 6, respectively, forperforming functions similar to contact 2R1.

The system is also provided with means for controlling the power controlcircuits to effect starting, acceleration, running and stopping of themotor. This means comprises a control circuit 36 common to the threepower control circuits. As shown in FIG. 1, control circuit 36 comprisespositive and negative power conductors 38 and 40, respectively, suppliedfrom a DC. source, positive conductor 38 being connected to the positiveside of this D.C. source through a manual onof switch 42 that isnormally open. The coil of relay IR is connected through a normallyclosed interlock contact 2R1 of relay 2R across conductors 38 and 40.Relay IR is a soft start control relay.

This common control circuit also comprises means for terminating theacceleration period This means comprises relay 2R having its coilconnected in series with the anode and cathode of an SCR (semiconductorcontrolled rectifier) 44 across conductors 38 and 40. A timer 46 isconnected across conductors 38 and 40 and its output is connected to thegate of the SCR to provide a firing pulse thereto when the timer timesout. Aresistor 48 is connected between the gate and cathode of the SCRto limit gate current. A clamping circuit comprising a resistor 50 and aunidirectional diode 52 in series is connected from the timer to theanode of the SCR and through the latter to negative conductor 40. Thisclamping circuit clamps the timer so that it will not continue pulsingafter it has fired the SCR into conduction, as hereinafter described inconnection with FIG. 2. Diode 52 blocks current flow through the relaycoil into the timer.

While timer 46 may take various forms, a circuit usable in FIG. 1 isshown in FIG. 2. This timer circuit comprises a bridge having a resistor54 and a capacitor 56 in one branch forming an RC timing circuit, withan output junction 58 therebetween. A pair of resistors 60 and 62 form aparallel branch of the bridge circuit having a control junction 64therebetween. Positive supply voltage is supplied to the upper side ofthis bridge through a resistor 66 while the lower side thereof isconnected directly to the negative side of the DC source. A programmableunijunction transistor PUT has its anode A connected to junction 58 andhas its gate G connected to junction 64. Cathode C of the PUT is theoutput. The clamping circuit is connected to junction 58 to shuntcapacitor 56 when the SCR is fired into conduction.

The timer operates to supply a current pulse from cathode C of the PUT apredetermined time interval after voltage is applied to the timer. Forthis purpose, following application of voltage, current flows throughresistors 66 and 54 to charge capacitor 56. Current also flows throughresistors 66, 60 and 62 whereby a predetermined voltage is applied fromjunction 64 to the gate of the PUT. When the voltage on capacitor 56that is applied to the anode exceeds the gate voltage, the PUT triggersinto conduction. As a result, capacitor 56 discharges through theanode-cathode junction thereof to apply a current pulse to the gate ofthe SCR. This fires the SCR into conduction for purposes hereinafterdescribed. I

OPERATION When it is desired to start the motor, switch 42 is closed. Asa result, relay 1R is energized to close contact lRll and correspondingcontacts in circuits 4 and 6. The operation of circuit 2 will bedescribed with reference to curve A in FIG. 3(c)and it will be seen thatcircuits 4 and 6 operate in a similar manner in their respective phasesas shown by curves B and C, respectively, in FIG. 3(b) and (a).

Upon closure of contact lRl, current flows to charge capacitor 28 at arate selected by adjustment of resistor 24. Resistor 26 provides anapproximate rate from which the charging current flow can be adjusted.When the voltage on capacitor 28 reaches a predetermined value, diac 30triggers into conduction and a pulse of current flows into the gate oftriac 8 to fire the triac into conduction. This charging of thecapacitor and triggering of the diac occurs symmetrically on eachhalf-cycle of phase A of the three-phase voltage. Each time the currentin the triac goes to or near zero, the triac stops conducting and mustbe refired into conduction on the next half-cycle of source voltage.This RC delay circuit, in effect, delays firing of the triac so that thelatter remains non-conducting for a predetermined period of time duringeach half-cycle.

This is shown by curve A in FIG. 3(c). When the current shown by curveI,, in FIG. 3(d) decreases to zero, this being the current flowingthrough conductor A to the motor, triac 8 turns off at point I so thatno current flows to the motor from line Ll. Triac 8 will stay off untilrefired into conduction at point Ll. Triac 8 will stay off until refiredinto conduction at point L Curve Ix shows generally how much greater thecurrent would be on each half-cycle without the soft start feature.

When the triac ceases conduction, it immediately transfers from a lowimpedance path to a near infinite impedance. In the three-phase system,a voltage V1 results across the triac which is the difference betweenthe instantaneous voltage on the line side of the triac and theresultant voltage on the load side of the open triac produced by thedivided difference of the values of the voltages impressed on the othertwo sides. As shown in FIG. 3(0), the voltage V1 on conductor A will beone-half (divided) the difference between the voltage of the other twophases B and C shown in dotted lines. The voltages of phases 3' and Care combined in motor winding BC and the resultant is divided intoone-half by windings AB and AC to apply a voltage on conductor A. Thisvoltage is further altered by any motor-generated voltages to providevoltage V1. This voltage wave form is shown in FIG. 3(e).

This voltage across the triac charges capacitor 28 until the voltagethereon reaches the trigger value of diac 30 at point 1,, in FIG. 3(d)atwhich time the charge on the capacitor isdumped-into the gate of thetriac clue to the diacs negative resistance characteristic. At thistime, the triac is fired into conduction, switching current on againthrough this line. In this state, the voltage across the triac dropsdown near zero as shown by the curve in FIG. 3(e). This action occurs onevery half-cycle, beginning each time the current through the triacdecreases to zero.

The same action occurs on each of the other two lines, except at phasedifferences. This action follows the rotation of the three-phase linesynchronously.

This action requires that only one triac be off at any given time sothat there is always a current path through the motor windings.

Further insight into the operation can be derived by carefully analyzingthe wave forms shown in FIG. 3. Although a delta load (no neutral) isbeing used, the voltage wave forms applied to the motor windings withrespect to a source neutral help considerably in graphically analyzingthe operation of the system. For instance, the voltage that occursacross the triac when it crosses through zero current can be seen to bethe difference between the line voltage on the one side and the combinedvoltages of the other two lines divided across the motor windings on theother side. Zero current crossover does not correspond to zero voltageacross the triac due to the inductive nature of the load as can be seenfrom the curves in FIG. 3(a) and (d). The dv/dt circuit consisting ofresistor and capacitor 12 across the triac and resistor 34 (inconjunction with the load inductance) similarly across the triac, andsimilar circuits across the other two triacs, help to eliminate falsecommutation due to the rapid voltage rise at point I as shown by thecurves in FIG. 3(a) and d).

Returning to the operation of common control circuit 36, it will berecalled that switch 42 was closed to energize relay 1R. This switchalso applies DC. power to timer 46 to start this timer timing. When thetimer times out, it fires SCR 44 into conduction to energize relay 2R.This relay closes its contact 2R1 in circuit 2 and similar contacts incircuits 4 and 6. Contact 2R1 completes a triac firing circuit throughresistor 32 in shunt of the time delay firing circuit. This firingcircuit fires the triac into conduction at the beginning of eachhalf-cycle to apply full voltage to the motor for running.

As will be apparent, timer 46 is set to time the motor acceleration atreduced voltage for soft start for a period of time sufficient to allowthe motor to reach full or near running speed as required to avoidapplication of excessive torque on the drive.

The curve BC in FIG. 3(f) shows how much the line to line power to themotor is reduced, this being a curve showing the voltage on conductor Bwith respect to conductor C.

The system provides excellent symmetry of the line current wave forms, afactor that is extremely important since any lack of symmetry wouldprovide D.C. components which would cause braking effects, very hightransient currents and rough operation.

,The width of the notch wave form, notch V1 in FIG. 3(a), is adjustablefrom a full on, zero degree notch, up to some-what below 60 wide perone-half cycle. This is done by adjustment of resistor 24 in each powercontrol circuit 2, 4 and 6. Observation of the wave forms shows thatthat much adjustment is possible while insuring that at least two linesare conducting at any given period of time. This available range isusually quite adequate to provide a sufficient span of power adjustmentfor most applications.

Use of this invention is not limited to motor or inductive loads.

Although the exemplary system disclosed is shown as applied to startinga motor, it is conceivable that such a phase controlling technique couldbe used as a continuously connected speed control or power controlsystem, with the understanding that its adjustment range is limited.

While a triac has been shown, it will be apparent that other A.C.switching means such as reverse-parallel connected SCRs could be used inplace thereof for other power applications with suitable means ofisolating the diac such as with a pulse transformer.

While the system hereinbefore described is effectively adapted tofulfill the objects stated, it is to be understood that the invention isnot intended to be con-' fined to the particular preferred embodiment ofsoft start A.C. motor control disclosed, inasmuch as it is susceptibleof various modifications without departing from the scope of theappended claims.

I claim:

1. A soft start polyphase control system comprising:

a polyphase A.C. supply source;

a polyphase A.C. load device having terminals;

a plurality of gating type bidirectional switching means;

means comprising power lines connecting said plurality of gating typebidirectional switching means between a plurality of the phases of saidsource and a plurality of said load terminals, respectively;

a plurality of time-delay firing circuits connected for energizationacross the respective gating type bidirectional switching means, eachsaid firing circuit being operable when the current in the associatedgating type bidirectional switching means decreases to zero to delayrefiring of the latter for a predetermined adjustable time intervalthereby to interrupt for time intervals cyclically the current flow tothe load device to reduce the power for soft starting;

a plurality of no-delay firing circuits connected for by-passing therespective time-delay firing circuits, each being operable to fire therespective gating type bidirectional switching means into conduction atthe beginning of each voltage half-cycle applied to the latter therebyto apply full power to the load device;

and control means common to said time-delay and no-delay firing circuitscomprising:

means for initiating operation of the system including switching meansresponsive thereto for rendering said time-delay firing circuitsoperative thereby to apply reduced power to the load device for softstarting;

timing means responsive to said initiation of operation of the system;

and means responsive to said timing means after a predetermined timeinterval for rendering'said nodelay firing circuits operative thereby toapply full power to the load device.

2. The invention defined in claim 1, wherein said soft start polyphasecontrol system also comprises:

a plurality of dv/dt networks connected across the respective gatingtype bidirectional switching means.

3. The invention defined in claim 2, wherein said soft start polyphasecontrol system further comprises:

a plurality of resistors connected cross the respective time-delayfiring circuits so as to be in parallel with the respective gating typebidirectional switching means when the associated time-delay firingcircuit is rendered operative and to be disconnected by said switchingmeans at other times.

4. The invention defined in claim 3, wherein said polyphase A.C. loaddevice comprises:

a delta connected A.C. motor having three phase terminals that areenergized through three of said gating type bidirectional switchingdevices from three phases of said A.C. supply source, respectively.

5. The invention defined in claim 1, wherein said time-delay firingcircuits comprise:

means for adjusting said delayed symmetrical refiring of each gatingtype bidirectional switching device from zero delay (full on) to a phaseangle short of any overlapping with a similar current interruption timeinterval in any of the other phases of the polyphase system.

6. The invention defined in claim 1, wherein each said time-delay firingcircuit comprises:

an RC time-delay circuit connected across the gating type bidirectionalswitching means;

and a symmetrical trigger diode connected between said RC time delaycircuit and the gate of said gating type bidirectional switching means.

7. The invention defined in claim 1, wherein said soft start polyphasecontrol system also comprises:

long conductors connecting said time-delay and nodelay firing circuitsfrom remote locations to the respective gating type bidirectionalswitching means; and a noise voltage suppressor circuit connected toeach gating type bidirectional switching means to suppress any noisevoltages that may be picked up by' said long conductors.

8. A soft start control system for controlling energization of athree-phase delta-connected or ungrounded Y-connected A.C. motor from athree-phase A.C. supply source comprising:

three power lines each including a triac connecting the source to themotor;

a time delay firing circuit energized across each said triac for firingthe associated triac into conduction; and control means common to saidtriacs comprising:

means operable after a predetermined acceleration on-off means operableto start and stop the motor;

means responsive to operation of said on-off means to start the motorfor rendering said time-delay firing circuits effective to controlapplication of reduced power to the motor;

and means operable after a predetermined acceleration period forcontrolling said triacs to apply full line voltage to themotor.

9. The invention defined in claim 8, wherein each said time-delay firingcircuit comprises:

an RC circuit connected for energization from across the anodes of therespective triac;

and a symmetrical trigger diode connected from the junction of said RCcircuit to the gate of the associated triac.

10. The invention defined in claim 9, wherein said soft start controlsystem also comprises:

a dv/dt network comprising a resistor and a capacitor connected acrosseach said triac;

and a damping circuit connected across each RC time-delay firingcircuit.

11. The invention defined in claim 10, wherein said damping circuitcomprises:

a resistor connected across each said time-delay firing circuit wherebythis resistor becomes connected across the associated triac when saidtimedelay firing circuit is rendered effective.

12. The invention defined in claim 11, wherein said P riod forcontrolling said triacs comprises:

a timer for timing a predetermined time interval from the time ofoperation of said on-off means;

and switching means responsive to said timer timing out at the end ofsaid predetermined time interval for shunting each time-delay firingcircuit to cause full line voltage firing of said triacs.

1. A soft start polyphase control system comprising: a polyphase A.C.supply source; a polyphase A.C. load device having terminals; aplurality of gating type bidirectional switching means; means comprisingpower lines connecting said plurality of gating type bidirectionalswitching means between a plurality of the phases of said source and aplurality of said load terminals, respectively; a plurality oftime-delay firing circuits connected for energization across therespective gating type bidirectional switching means, each said firingcircuit being operable when the current in the associated gating typebidirectional switching means decreases to zero to delay refiring of thelatter for a predetermined adjustable time interval thereby to interruptfor time intervals cyclically the current flow to the load devicE toreduce the power for soft starting; a plurality of no-delay firingcircuits connected for by-passing the respective time-delay firingcircuits, each being operable to fire the respective gating typebidirectional switching means into conduction at the beginning of eachvoltage halfcycle applied to the latter thereby to apply full power tothe load device; and control means common to said time-delay andno-delay firing circuits comprising: means for initiating operation ofthe system including switching means responsive thereto for renderingsaid time-delay firing circuits operative thereby to apply reduced powerto the load device for soft starting; timing means responsive to saidinitiation of operation of the system; and means responsive to saidtiming means after a predetermined time interval for rendering saidno-delay firing circuits operative thereby to apply full power to theload device.
 2. The invention defined in claim 1, wherein said softstart polyphase control system also comprises: a plurality of dv/dtnetworks connected across the respective gating type bidirectionalswitching means.
 3. The invention defined in claim 2, wherein said softstart polyphase control system further comprises: a plurality ofresistors connected cross the respective time-delay firing circuits soas to be in parallel with the respective gating type bidirectionalswitching means when the associated time-delay firing circuit isrendered operative and to be disconnected by said switching means atother times.
 4. The invention defined in claim 3, wherein said polyphaseA.C. load device comprises: a delta connected A.C. motor having threephase terminals that are energized through three of said gating typebidirectional switching devices from three phases of said A.C. supplysource, respectively.
 5. The invention defined in claim 1, wherein saidtime-delay firing circuits comprise: means for adjusting said delayedsymmetrical refiring of each gating type bidirectional switching devicefrom zero delay (full on) to a phase angle short of any overlapping witha similar current interruption time interval in any of the other phasesof the polyphase system.
 6. The invention defined in claim 1, whereineach said time-delay firing circuit comprises: an RC time-delay circuitconnected across the gating type bidirectional switching means; and asymmetrical trigger diode connected between said RC time delay circuitand the gate of said gating type bidirectional switching means.
 7. Theinvention defined in claim 1, wherein said soft start polyphase controlsystem also comprises: long conductors connecting said time-delay andno-delay firing circuits from remote locations to the respective gatingtype bidirectional switching means; and a noise voltage suppressorcircuit connected to each gating type bidirectional switching means tosuppress any noise voltages that may be picked up by said longconductors.
 8. A soft start control system for controlling energizationof a three-phase delta-connected or ungrounded Y-connected A.C. motorfrom a three-phase A.C. supply source comprising: three power lines eachincluding a triac connecting the source to the motor; a time delayfiring circuit energized across each said triac for firing theassociated triac into conduction; and control means common to saidtriacs comprising: on-off means operable to start and stop the motor;means responsive to operation of said on-off means to start the motorfor rendering said time-delay firing circuits effective to controlapplication of reduced power to the motor; and means operable after apredetermined acceleration period for controlling said triacs to applyfull line voltage to themotor.
 9. The invention defined in claim 8,wherein each said time-delay firing circuit comprises: an RC circuitconnected for energization from across the anodes of the respectivetriac; and a Symmetrical trigger diode connected from the junction ofsaid RC circuit to the gate of the associated triac.
 10. The inventiondefined in claim 9, wherein said soft start control system alsocomprises: a dv/dt network comprising a resistor and a capacitorconnected across each said triac; and a damping circuit connected acrosseach RC time-delay firing circuit.
 11. The invention defined in claim10, wherein said damping circuit comprises: a resistor connected acrosseach said time-delay firing circuit whereby this resistor becomesconnected across the associated triac when said time-delay firingcircuit is rendered effective.
 12. The invention defined in claim 11,wherein said means operable after a predetermined acceleration periodfor controlling said triacs comprises: a timer for timing apredetermined time interval from the time of operation of said on-offmeans; and switching means responsive to said timer timing out at theend of said predetermined time interval for shunting each time-delayfiring circuit to cause full line voltage firing of said triacs.